Radiographic imaging employs a source of penetrative radiation, such as an x-ray tube, and a means responsive to x-rays to indicate characteristics of a pattern of x-rays emergent from a subject when placed in the x-ray beam path between the source and the x-ray sensitive means. The x-ray sensitive means can take many forms, such as an x-ray screen, for converting x-rays to light, overlying a piece of light and x-ray sensitive film for producing a shadow graphic picture of the internal structure or condition of the subject. More recently, radiographic detectors have been embodied by cellularized detectors of various types, defining an area expanse and including many individual detectors each responsive to radiation incident upon its particular zone. See for example, U.S. Pat. No. 4,626,688, issued on Dec. 2, 1986 to Barnes, which is hereby expressly incorporated by reference.
Other techniques for imaging the internal structure or condition of a subject by use of penetrative radiation include computerized tomographic scanning and nuclear camera imaging, both of which are well known in the art, and which will not be discussed in detail here.
Some radiation imaging systems employ automatic exposure control. In a traditional means of automatic exposure control radiography, for example, a feedback signal is produced which is a function of the level of x-ray exposure taking place over time. An x-ray sensor, sometimes called a "paddle", is mounted in the vicinity of the x-ray sensitive means such as near or on a cassette holding a screen/film assembly. The sensor sometimes has comprised a photomultiplier tube which produces a voltage which is a function of the instantaneous level of x-ray energy incident upon the receiving face of the tube. Integrating circuitry is provided and coupled to the photomultiplier tube, which, in response to the tube's reaction to x-rays, produces a voltage signal which is a function of the time integral of x-ray energy which has been incident on the tube during the exposure.
This integrated signal forms a ramp signal which is used to control the exposure by comparing it to a fixed threshold reference value. The exposure is allowed to run until the value of the ramp signal exceeds the reference value, or until a maximum predetermined interval of time has elapsed, whichever comes first. This predetermined time is commonly referred to as a "backup time". The backup time value is often set to a time of several seconds or more, a time of five (5) or six (6) seconds being common.
Alternately, it is also known to preprogram a radiographic system to terminate the exposure only upon lapse of a certain predetermined exposure time less than the backup time.
The ramp signal/backup time exposure control technique suffers from the disadvantage that, if radiation reaching the sensor or paddle is insufficient to increase the integrated radiation indicating ramp signal to the predetermined exposure termination threshold level prior to expiration of the backup time, the exposure will continue until the backup time runs out, without regard to the fact that, if the ramp signal is not increasing with sufficient speed, a poor exposure is likely being made. Thus, the patient is subjected to a dose of radiation for the entire backup time, only to learn later that the exposure was inadequate and would have to be performed again after corrective measures.
At least two conditions can contribute to the failure of sufficient increase in the integrated radiation accumulation ramp signal. First, that ramp signal will not increase with sufficient speed if the screen/film cassette is not properly aligned in the x-ray beam from the source, since the radiation sensing paddle is usually mounted on or quite near the cassette itself. Thus, if the source is moved to a position over the patient's chest, and the cassette moved to a location under his abdomen, actuation of the source will cause the propagation of x-rays through the patient's chest for the full backup period of several seconds without yielding any picture at all. The dose will thus have been wasted, and the patient would have to be re-exposed to the radiation this time with the cassette properly aligned.
Another condition which can result in excessively slow ramp signal buildup is where the radiation system, while properly aligned, is not adjusted for proper radiation emission level. If, for example, a large patient were to be imaged, but the source was preset for delivering radiation of only sufficient intensity to image a small patient's body, the radiation might likely continue for the entire backup time without raising the ramp signal to the threshold level to induce exposure termination. Under these circumstances, it is likely that the film obtained will be insufficiently exposed, which will, as in the previous case of misalignment, result in the necessity for re-exposing the patient to more radiation to make a second exposure following a failed first exposure which endured for the entire length allowed by the backup timer.
Details of known automatic exposure control are illustrated in the U.S. Patent to Slagle and in the U.S. patent application to Griesmer, both fully identified below and hereby expressly incorporated by reference.
An object of this invention is to provide exposure control for radiographic imaging which monitors the cumulative progress of an ongoing exposure and aborts the exposure automatically in the event that the exposure appears from its early progress to be a likely failure.